MAXI J1820+070: Black Hole X-ray Binary

• MAXI J1820+070 is a black hole X-ray binary transient discovered in 2018, renowned for its bright outburst and extensive multiwavelength coverage.
• Its comprehensive monitoring enabled precise dynamical mass confirmation and detailed studies of state transitions, compact jets, and disc re-brightenings.
• Spectral-timing analyses of the source provide critical insights into accretion-ejection coupling and calibrate models of jet energetics and disc instability.

Searching arXiv for recent and foundational papers on MAXI J1820+070 to support the article. MAXI J1820+070, also known as ASASSN-18ey, is a black hole X-ray binary transient and low-mass microquasar discovered as an optical transient on 2018 March 6 and identified in X-rays by MAXI on 2018 March 11. Its 2018 outburst was exceptionally bright, remained active across radio, optical, ultraviolet, and X-ray bands for more than four years, and was monitored with an unusual degree of cadence and simultaneity. As a result, the source has become a benchmark system for dynamical black-hole confirmation, broadband spectral-timing, compact-jet diagnostics, state-transition physics, and large-scale ejecta studies (Torres et al., 2019, Bright et al., 15 Jul 2025).

1. Discovery, outburst chronology, and source class

MAXI J1820+070 was recognized early as a black hole low-mass X-ray binary in a major outburst. The 2018 event reached about $4$ Crab in the $15\!-\!50$ keV band as seen by Swift/BAT, stayed in the hard state for almost $4$ months, underwent a rapid hard-to-soft transition in July 2018, entered the soft state, and later hardened again in September 2018 before quiescence. The source traced the typical q-shaped hardness-intensity-diagram evolution expected for black-hole transients (Hoang et al., 2019).

Longer-baseline monitoring extended this canonical picture. The full outburst history from MJD 58189 to 59792 shows a rising hard state, one hard-to-soft transition, a soft state, a return to the hard state, and then three hard-state-only re-brightenings denoted R1, R2, and R3. The post-soft-state hard phase is labeled R0 and is morphologically similar to the later re-brightenings. Swift/XRT coverage continued to MJD 59735, and true quiescence was reached only in June 2023 (MJD 60106) (Bright et al., 15 Jul 2025).

This observational record matters because the source combined high intrinsic luminosity, low distance, and dense multiwavelength coverage. A plausible implication is that MAXI J1820+070 is valuable not only as a single black-hole transient, but as a calibration case for comparing compact jets, discrete ejecta, irradiated discs, and repeated mini-outbursts within one system.

2. Orbital solution, donor star, and dynamical black-hole confirmation

Torres et al. dynamically confirmed the compact object through time-resolved optical spectroscopy obtained during the decline toward quiescence. Cross-correlation of 21 spectra against late-type templates yielded a sinusoidal donor-star radial-velocity curve with $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$ and $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$, implying a mass function $f(M)=5.18 \pm 0.15\,M_\odot$. Because this value is already above the maximum allowed neutron-star mass, the unseen primary is dynamically confirmed as a black hole (Torres et al., 2019).

Parameter	Value	Interpretation
$P_{\rm orb}$	$0.68549 \pm 0.00001\,\mathrm{d}$	Spectroscopic orbital period
$K_2$	$417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$	Donor semi-amplitude
$15\!-\!50$0	$15\!-\!50$1	Dynamical lower limit on compact-object mass
Donor type	K3–K5	Late-type Roche-lobe-filling companion
Donor flux fraction	$15\!-\!50$2 at $15\!-\!50$3 Å	Disc-diluted stellar absorption
Inclination limits	$15\!-\!50$4	Grazing disc eclipse and no X-ray eclipses

The donor star is most consistent with a K3–K5 star and contributes only about $15\!-\!50$5 of the total light in the $15\!-\!50$6 Å band, the rest being dominated by the accretion disc. Torres et al. also reinterpreted the outburst photometric periodicity $15\!-\!50$7 as a superhump because it is $15\!-\!50$8 longer than the spectroscopic orbital period. Using the period excess, they inferred a binary mass ratio $15\!-\!50$9. The H$4$0 equivalent width increases sharply near inferior conjunction, which they interpreted as a grazing eclipse of the accretion disc and hence as evidence for $4$1, while the absence of X-ray eclipses implies $4$2; together these bounds gave an initial black-hole mass range of $4$3 (Torres et al., 2019).

Atri et al. later obtained a model-independent radio parallax $4$4, corresponding to $4$5. With the proper motions of the approaching and receding ejecta, they derived a jet inclination $4$6, a jet speed $4$7, and a revised black-hole mass $4$8. They also used the systemic radial velocity $4$9, proper motion, and parallax to infer a kick distribution with median about $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$0, with 5th and 95th percentiles of $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$1 and $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$2 (Atri et al., 2019).

3. Hard-state accretion flow, reflection, and spectral-timing phenomenology

Broadband X-ray studies established MAXI J1820+070 as a canonical but unusually well constrained hard-state system. AstroSat observations with SXT and LAXPC showed that the $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$3 keV spectrum was well modeled by a multicolor disk black-body, thermal Comptonization, and reflection. In that state the continuum had $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$4 and the disc was cool, with $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$5. The same data revealed strong broad-band aperiodic variability, energy-dependent hard lags, and a quasi-periodic oscillation at $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$6; the lag and rms behavior were reproduced reasonably well by a single-zone stochastic propagation model, supporting inwardly propagating fluctuations through a compact Comptonizing region (Mudambi et al., 2020).

NuSTAR monitoring of the initial hard state added a geometrical constraint. The broad reflection spectrum remained remarkably stable across eight hard-state epochs, with an average inner disc radius $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$7, even though the low-frequency break and QPO frequency increased by about a factor of $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$8. At the same time, the broad iron line and Compton hump were well described by a two-component extended lamp-post corona, and the coronal temperature rose from roughly $P_{\rm orb}=0.68549 \pm 0.00001\,\mathrm{d}$9 keV to $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$0 keV. This combination suggested that the variability timescales were governed by coronal conditions rather than solely by the disc inner edge (Buisson et al., 2019).

Joint AstroSat and NuSTAR spectroscopy during the hard state sharpened the same picture. Reflection fits required a cool disc with $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$1 keV, a short lamp-post height $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$2, a hot primary corona with $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$3 keV, and a second Comptonization component at $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$4 keV. The preferred interpretation was an inhomogeneous corona whose temperature decreases with increasing height, with super-solar iron abundance and a narrow distant reflection component also required by the data (Chakraborty et al., 2020).

A later spectral-timing analysis combining NICER and Insight-HXMT showed that a propagating-fluctuation model with fixed local spectral shapes fails when extrapolated to higher energies. Allowing spectral pivoting, rather than pure normalization changes, reproduced the observed suppression of high-energy variability much more effectively and yielded a statistically good joint fit to the $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$5 keV spectrum and variability. The derived propagation timescale is slower than predicted by a magnetically arrested disc, despite the source showing a strong jet, and the QPO is therefore most easily explained as an extrinsic modulation of the flow, such as Lense-Thirring precession, rather than as an additional jet spectral-timing component (Kawamura et al., 2022).

4. Compact jets, discrete ejecta, and the accretion-ejection connection

MAXI J1820+070 became a key source for timing the onset of jet ejection. During the rapid hard-to-soft transition, NICER recorded a switch from type-C to type-B QPOs at MJD $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$6, accompanied by a drop in broadband X-ray noise and a brief flare in the $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$7 keV band. AMI-LA then observed a strong $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$8 GHz radio flare whose inferred start time, MJD $K_2=417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$9, was about $f(M)=5.18 \pm 0.15\,M_\odot$0 hr after the type-B QPO first appeared. The close sequencing of the QPO transition, hard X-ray flare, and radio event was presented as the strongest observational evidence to date for a link between the appearance of type-B QPOs and the launch of discrete jet ejections (Homan et al., 2020).

High-resolution radio imaging showed that the flare was associated with bipolar relativistic ejecta rather than continued compact-jet emission. Bright et al. found that the isolated radio flare lasted about $f(M)=5.18 \pm 0.15\,M_\odot$1, began at MJD $f(M)=5.18 \pm 0.15\,M_\odot$2, and occurred while the core jet was suppressed by a factor of over $f(M)=5.18 \pm 0.15\,M_\odot$3. The approaching ejecta were monitored for over $f(M)=5.18 \pm 0.15\,M_\odot$4 days and out to a maximum angular separation of $f(M)=5.18 \pm 0.15\,M_\odot$5. In the ballistic model, the proper motions are $f(M)=5.18 \pm 0.15\,M_\odot$6 and $f(M)=5.18 \pm 0.15\,M_\odot$7, implying a lower limit $f(M)=5.18 \pm 0.15\,M_\odot$8; the radio parallax distance $f(M)=5.18 \pm 0.15\,M_\odot$9 kpc was argued to rule out the constant-deceleration model, so the ballistic interpretation was preferred. Their energy estimate for the resolved approaching ejecta, $P_{\rm orb}$0, is orders of magnitude larger than the flare-based estimate $P_{\rm orb}$1, implying that state-transition radio flares can systematically underestimate jet energetics (Bright et al., 2020).

A subsequent VLBA re-analysis showed that more than one ejection occurred. An earlier slow component, designated A, has proper motion $P_{\rm orb}$2, an ejection date MJD $P_{\rm orb}$3, and an ejection duration of roughly $P_{\rm orb}$4 hours. It was launched $P_{\rm orb}$5 hours before the beginning of the rise of the radio flare and $P_{\rm orb}$6 hours before the type-C to type-B QPO switch. The later fast component has revised speed $P_{\rm orb}$7, the slow component $P_{\rm orb}$8, and the updated jet inclination is $P_{\rm orb}$9. In that interpretation, the slow component is predominantly responsible for the radio flare and is likely linked to the QPO transition, while no definitive launch signature was identified for the fast ejecta (Wood et al., 2021).

The system also supplied a rare direct timing-based measurement of the compact hard-state jet. Simultaneous ten-band fast timing from radio to X-rays over a $0.68549 \pm 0.00001\,\mathrm{d}$0-hour interval measured lags ranging from hundreds of milliseconds between X-ray and optical bands to minutes between sub-mm and radio bands. Modeling these lags, fluxes, and power-spectral breaks yielded a highly relativistic jet with $0.68549 \pm 0.00001\,\mathrm{d}$1, a confined opening angle $0.68549 \pm 0.00001\,\mathrm{d}$2 deg, and power equivalent to $0.68549 \pm 0.00001\,\mathrm{d}$3. The same analysis placed constraints on jet composition and magnetic field strength, including $0.68549 \pm 0.00001\,\mathrm{d}$4 in the innermost jet base region and a jet that cannot be proton dominated (Tetarenko et al., 2021).

At larger scales, Chandra, VLA, and MeerKAT detected X-ray sources associated with the radio jets moving at relativistic velocities. The south jet was spatially resolved with length $0.68549 \pm 0.00001\,\mathrm{d}$5, transverse size $0.68549 \pm 0.00001\,\mathrm{d}$6, and opening angle $0.68549 \pm 0.00001\,\mathrm{d}$7. Broadband radio-to-X-ray spectra are consistent with optically thin synchrotron emission from shock-accelerated particles, with inferred electron energies above $0.68549 \pm 0.00001\,\mathrm{d}$8 TeV. The minimal internal energy estimated from the X-ray observations is $0.68549 \pm 0.00001\,\mathrm{d}$9 erg, and the detailed equipartition calculation gives $K_2$0 erg. This reinforced the conclusion that much of the jet energy is released through late-time interaction with the surrounding medium rather than in the compact radio flare alone (Espinasse et al., 2020).

5. Optical, ultraviolet, and disc-geometry diagnostics

Optical timing showed that the non-X-ray emission is not produced by a single mechanism. Simultaneous HiPERCAM and NICER observations during the hard-state rise revealed intense red flaring with flux changes of roughly a factor of $K_2$1 down to $K_2$2 ms. Cross-correlation analysis found a broad anti-correlation on timescales of seconds, a narrow positive correlation at a lag of about $K_2$3 ms with the optical lagging the X-rays, and an additional $K_2$4 s lag feature that may be due to disc reprocessing. The sub-second lag increases with optical wavelength and is approximately constant over Fourier frequencies of $K_2$5 Hz, which is consistent with an origin in the inner accretion flow and jet base within $K_2$6 gravitational radii (Paice et al., 2019).

Long-baseline optical, ultraviolet, and X-ray monitoring extended these constraints to the full outburst and later rebrightenings. During the rebrightening process, the optical rise preceded the X-ray rise by $K_2$7 days. The spectra are characterized by blue continua and emission from the Balmer series, He I, He II, and a broad Bowen blend. The pseudo equivalent widths of the emission lines show anticorrelations with the X-ray flux measured at comparable phases because the optical continuum increasingly suppresses the equivalent widths, but the absolute line fluxes of H$K_2$8 and He II $K_2$9 correlate positively with X-ray flux, favoring irradiation as the main driver of line emission. Near the X-ray peak, the full widths at half maximum stabilize at $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$0 Å for H$417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$1 and $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$2 Å for He II $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$3, corresponding to line-forming radii of $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$4 and $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$5, respectively. When the X-ray flux drops dramatically at $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$6 days while the optical and UV flux change only modestly, the observations suggest that viscous dissipation in the disc contributes in addition to irradiation (Sai et al., 2021).

MAXI J1820+070 also showed unusually large optical orbital-scale modulations. AAVSO light curves reveal a modulation with amplitude $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$7 mag, often around $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$8 mag, that appears about $417.7 \pm 3.9\,\mathrm{km\,s^{-1}}$9 days after the outburst began and evolves from the superhump period $15\!-\!50$00 toward a later period $15\!-\!50$01 close to the orbital period $15\!-\!50$02. The modulation begins as the Swift/BAT hard X-ray light curve rises toward a secondary maximum and peaks $15\!-\!50$03 days before the radio flare and jet ejection. Because the amplitude is much larger than expected for a standard tidal superhump alone, the preferred interpretation is a warped, irradiation-driven outer disc whose visibility depends on a change in the inner hard-X-ray geometry (Thomas et al., 2021).

6. Spin determinations, gamma-ray constraints, and long-term significance

Published spin estimates for MAXI J1820+070 are method-dependent and remain debated. Insight-HXMT continuum-fitting during the high soft state found an apparent evolution of the inferred spin, which is physically impossible on outburst timescales. That evolution coincides with a drop in hardness ratio, a drop in non-thermal luminosity, and an increase in reflection fraction around MJD $15\!-\!50$04, and was attributed to changes in the inner disc and corona rather than true spin change. Using only the epoch where $15\!-\!50$05 is stable and $15\!-\!50$06, the constrained spin is $15\!-\!50$07. In that framework, the strong jet despite low spin suggests that jet power is driven mainly by accretion-disc processes rather than by black-hole spin extraction (Guan et al., 2020).

A timing-based estimate from NICER leads to a very different value. By fitting the evolution of type-C QPOs and broad-band noise with the Relativistic Precession Model, the spin was inferred to be $15\!-\!50$08, with the characteristic variability radius spanning roughly $15\!-\!50$09. The authors explicitly noted that model systematics likely make this estimate a lower bound rather than a fully assumption-free determination (Bhargava et al., 2021).

A later NuSTAR soft-state study also favored a high-spin solution. Broadband $15\!-\!50$10 keV modeling of five soft-state observations found $15\!-\!50$11, often near $15\!-\!50$12 in epoch-by-epoch fits, together with a mid-soft-state decline in the inner disc temperature, a modest increase in the inferred inner disc radius up to $15\!-\!50$13, and a soft X-ray excess below $15\!-\!50$14 keV well fit by a blackbody with $15\!-\!50$15. In that interpretation, the soft excess arises from a warm corona beyond $15\!-\!50$16 rather than from the plunge region. This suggests that the spin discrepancy is tied to different assumptions about whether the disc reaches the ISCO and how the soft excess is modeled (Papavasileiou et al., 13 Mar 2026).

Gamma-ray observations add an external constraint on non-thermal outflow models. A combined H.E.S.S., MAGIC, VERITAS, and Fermi-LAT campaign during the 2018 outburst detected no significant gamma-ray emission. The upper limits imply that, if a high-energy or very-high-energy gamma-ray emitting region exists during the state transitions, it should lie at distances of roughly $15\!-\!50$17 cm from the black hole; in the hard state, predicted fluxes can be at most a factor of $15\!-\!50$18 below the Fermi-LAT upper limits under the assumptions adopted in the paper (Abe et al., 2022).

The long-term accretion-ejection record consolidates the broader importance of the source. Hard states define a Radio–X-ray–Optical activity plane with a high degree of correlation among the three wave bands, while the source shows hysteresis as it enters and exits the soft state. The fading hard state and the later re-brightenings are broadly consistent with modified disk instability models that include irradiation from the inner accretion disk. This makes MAXI J1820+070 a particularly complete case study of how compact jets, discrete ejecta, irradiated discs, and repeated hard-state re-brightenings fit together in a single black-hole X-ray binary (Bright et al., 15 Jul 2025).